(1) Field of the Invention
The present invention relates generally to integrated electronic circuits. More specifically, the present invention relates to an interconnect system for an integrated circuit and a method of fabricating the interconnect system.
(2) Description of the Related Art
Modern integrated circuits are generally made up of millions of active and passive devices such as transistors, capacitors, and resistors disposed onto a silicon wafer. These devices are initially isolated from one another, but are later interconnected together by an interconnect system to form functional circuits. The quality of the interconnect system drastically affects the performance and reliability of the fabricated circuit.
An interconnect system typically includes metal lines, spaced apart from each other, which interconnect the various active and passive devices found in a silicon wafer onto which the interconnect system had been deposited and fabricated. Insulating dielectric layers are deposited between the metal lines for isolating the metal lines from one another. Inherent in the structure of the interconnect system is a capacitance associated with the metal lines spaced apart from each other. As the capacitance is to first order inversely proportional with the distance between the metal lines, one way to reduce the capacitance between two lines would be to increase the space between the lines. However, this option is limited by the integration capacity of the integrated circuit.
A preferable way to reduce the capacitance between two lines of the interconnect system is to reduce the dielectric constant k of the dielectric material deposited between the metal lines, as the capacitance is directly proportional to the dielectric constant of the dielectric material between the metal lines. Typically, interconnect systems use a dielectric material such as Silicon Dioxide (SiO.sub.2). The dielectric constant of SiO.sub.2 is approximately 4. Other dielectric materials can be used such as Silicon Oxifluoride (SiOF) with a dielectric constant of 3-3.5, or Aerogel which is a foam with vacuum or air pores with a dielectric constant which can approach the dielectric constant of air, i.e., 1. The problem with dielectric materials having lower dielectric constants is that they may not adequately support the structure of the interconnect system as these materials have a relatively high Young's modulus which makes them relatively soft, especially for materials with the dielectric constant k&lt;2.
FIG. 1 shows a cross-sectional view of an interconnect system 100. The interconnect system 1 has a substrate 2 which typically includes active and passive electronic structures. An insulating layer 4 is deposited over substrate 2. The insulating layer 4 is made of a dielectric material which provides electrical insulation between the substrate 2 and any other electrically conductive layer deposited upon layer 4. Layer 4 could be, by way of non-limiting example, silicon dioxide (SiO.sub.2) either doped or undoped, silicon nitride (Si.sub.3 N.sub.4) or silicon oxinitride (SiO.sub.x N.sub.y). Following the deposition of the insulating layer 4 a metal layer is deposited thereon. Electrically conductive interconnect lines 10 are then formed by etching out selected portions of the metal layer. The electrically conductive interconnect lines 10 serve the purpose of interconnecting various electronic devices, active or passive, which reside in the substrate 2. The interconnect lines 10 are spaced apart from each other in order not to short circuit the various devices to which the interconnect lines are connected. The space between interconnect lines 10 is typically filled with a dielectric material 8 which is deposited upon interconnect lines 10 forming the pattern shown in FIG. 1. A second electrically conductive layer of interconnect lines 26 is formed over the dielectric layer 8 in a direction usually orthogonal to the direction of the interconnect lines 10. Continuing the above process, more insulating layers followed by conducting layers could be deposited to finally form the interconnect system.
The interconnect system illustrated in FIG. 1 uses SiO.sub.2 for the dielectric layer 8. While this type of material can provide the mechanical support necessary to withstand the stress of the structures deposited thereupon, the resulting large capacitance between lines, due to the relatively high dielectric constant of SiO.sub.2, makes this material less and less desirable in the high performance interconnect systems of the future.
FIG. 2 shows a cross sectional view of an interconnect system 200 wherein the dielectric layer 8 has been applied over interconnect lines 10 so as not to fill out completely the gaps between the interconnect lines 10 and, thus, to provide for air gaps 16 between the interconnect lines 10. However, due to the nature of the structure of the interconnect system and of the various processes for deposition of the dielectric layer 8, which include physical deposition, chemical vapor deposition, and plasma deposition, a certain amount of the dielectric is incorporated between the metal lines 10. As one can see, the dielectric material 8 is still deposited between the metal lines 10 leaving small voids 16. This is undesirable as the dielectric constant and thus the capacitance between the adjacent lines increases with the amount of dielectric material between the lines.
It is desirable to provide for an interconnect system wherein the space between the interconnect lines has a dielectric constant as low as possible and substantially uniform throughout that space.